Microwave Dielectric Heating of Drops in Microfluidic Devices

We present a technique to locally and rapidly heat water drops in microfluidic devices with microwave dielectric heating. Water absorbs microwave power more efficiently than polymers, glass, and oils due to its permanent molecular dipole moment that has a large dielectric loss at GHz frequencies. The relevant heat capacity of the system is a single thermally isolated picoliter drop of water and this enables very fast thermal cycling. We demonstrate microwave dielectric heating in a microfluidic device that integrates a flow-focusing drop maker, drop splitters, and metal electrodes to locally deliver microwave power from an inexpensive, commercially available 3.0 GHz source and amplifier. The temperature of the drops is measured by observing the temperature dependent fluorescence intensity of cadmium selenide nanocrystals suspended in the water drops. We demonstrate characteristic heating times as short as 15 ms to steady-state temperatures as large as 30 degrees C above the base temperature of the microfluidic device. Many common biological and chemical applications require rapid and local control of temperature, such as PCR amplification of DNA, and can benefit from this new technique.Comment: 6 pages, 4 figure

Human saliva and model saliva at bulk to adsorbed phases – similarities and differences

Human saliva, a seemingly simple aqueous fluid, is, in fact, an extraordinarily complex biocolloid that is not fully understood, despite many decades of study. Salivary lubrication is widely believed to be a signature of good oral health and is also crucial for speech, food oral processing and swallowing. However, saliva has been often neglected in food colloid research, primarily due to its high intra- to inter-individual variability and altering material properties upon collection and storage, when used as an ex vivo research material. In the last decade, colloid scientists have attempted designing model (i.e. ‘saliva mimicking fluid’) saliva formulations to understand saliva-food colloid interactions in an in vitro set up and its contribution on microstructural aspects, lubrication properties and sensory perception. In this Review, we critically examine the current state of knowledge on bulk and interfacial properties of model saliva in comparison to real human saliva and highlight how far such model salivary formulations can match the properties of real human saliva. Many, if not most, of these model saliva formulations share similarities with real human saliva in terms of biochemical compositions, including electrolytes, pH and concentrations of salivary proteins, such as α-amylase and highly glycosylated mucins. This, together with similarities between model and real saliva in terms of surface charge, has led to significant advancement in decoding colloidal interactions (bridging, depletion) of charged emulsion droplets and associated sensory perception in the oral phase. However, model saliva represents significant dissimilarity to real saliva in the lubricating properties. Based on in-depth examination of properties of mucins from animal sources (e.g. pig gastric mucins (PGM) or bovine submaxillary mucin (BSM)), we can recommend that BSM is currently the most optimal mucin source when attempting to replicate saliva based on surface adsorption and lubrication properties. Even though purification via dialysis or chromatographic techniques may influence various physicochemical properties of BSM, such as structure and surface adsorption, the lubricating properties of model saliva formulations based on BSM are generally superior and more reliable than PGM counterpart at orally relevant pH. Comparison of mucin-containing model saliva with ex vivo human salivary conditioning films suggests that mucin alone cannot replicate the lubricity of real human salivary pellicle. Mucin-based multi-layers containing mucin and oppositely charged polyelectrolytes may offer promising avenues in the future for engineering biomimetic salivary pellicle, however, this has not been explored in oral tribology experiments to date. Hence, there is a strong need for systematic studies with employment of model saliva formulations containing mucins with and without polycationic additives before a consensus on a standardized model saliva formulation can be achieved. Overall, this review provides a comprehensive framework on simulating saliva for a particular bulk or surface property when doing food oral processing experiments

Characterization of an Aerosol Shock Tube Facility for Heterogeneous Combustion Studies

Combustion is responsible for providing energy for many applications, especially in propulsion and rocket propellants. Shock tubes provide a controlled, repeatable means of studying combustion characteristics; although, most of these studies require the fuel in a mixture to exist in pure gas-phase. This makes it challenging to test low-vapor-pressure fuels that tend to remain in condensed form. Low-vapor-pressure fuels are commonly used in many combustion applications, making combustion studies of these fuels important. A method to study low-vapor-pressure fuels using a shock tube approach is to inject the fuel into the shock tube as tiny, uniformly-sized aerosol droplets. The sub-micron-sized aerosol droplets remain uniformly suspended in the shock tube prior to running the experiment. An incident shock wave vaporizes the liquid fuel droplets, then the reflected shock wave initiates ignition of the mixture. This study presents the characterization of an aerosol fuel injection method to the shock tube to study the combustion of low-vapor-pressure fuels. An aerosol generator was used to produce repeatable, uniformly-sized fuel droplets, and flow controllers were used to control and measure oxygen and argon dilution gas injected into the shock tube. A technique was developed to ensure consistent and repeatable aerosol fuel production rates over which calibration curves were found. This study presents the ignition delay times for C7H16 (ϕ = 1.0) at a pressure of 2.0 atm for temperatures from 1220 - 1427 K, C7H8 (ϕ = 1.0) at 1.9 atm over a temperature range of 1406 – 1791 K, and C12H26 (ϕ = 0.3) at 3.0 atm for the temperature range of 1293 – 1455 K. The ignition delay times for heptane and toluene were compared to the literature values at the same conditions and were found to be in good agreement. Laser extinction (visible laser at 632nm) was used to verify the presence of aerosol fuel droplets inside the shock tube for dodecane, but showed the heptane aerosol vaporized upon injection into the shock tube. Initial laser absorption (3.39 µm) measurements were also taken. This aerosol technique was found to successfully evaluate combustion effects of low-vapor-pressure fuels; however, was limited by the range of possible fuel concentrations. Further work needs to be performed on the verification of aerosol spatial uniformity and obtaining higher fuel concentrations

Ignition of a Liquid Hydrocarbon Containing Nano-Sized Aluminum using an Aerosol Shock Tube

An experimental approach has been taken to investigate nanoparticle additives in a liquid fuel using an aerosol shock tube. Aluminum nanoparticles have shown the potential to increase combustion efficiencies due to their high energy densities. Challenges exist, however, with nanoparticle suspension in liquid fuels. Nanoparticles have a tendency to agglomerate and thus prevent ignition. This study looked at the effect of aluminum nanoparticle additives to toluene. An aerosol shock tube approach was used to evaluate the combustion of a baseline toluene mixture and a toluene/Al nanoparticle mixture with 95% dilution in argon. An aerosol fuel injection method was used to ensure injection of the nanoparticle-doped fuel into the shock tube. The combustion of both the toluene and toluene/Al nanoparticles was determined over the temperature range of 1404 - 1790 K and a pressure around 1.9 atm behind the reflected shock wave. To determine the influence of aluminum nanoparticle additives, the ignition delay times were studied and the results from these experiments are reported. In general, the ignition delay times for temperatures below about 1700 K were not affected by the Al additives, but at higher temperatures the ignition delay times appears to be shorter than for neat toluene. © 2013 by the authors

Ignition Of A Liquid Hydrocarbon Containing Nano-Sized Aluminum Using An Aerosol Shock Tube

An experimental approach has been taken to investigate nanoparticle additives in a liquid fuel using an aerosol shock tube. Aluminum nanoparticles have shown the potential to increase combustion efficiencies due to their high energy densities. Challenges exist, however, with nanoparticle suspension in liquid fuels. Nanoparticles have a tendency to agglomerate and thus prevent ignition. This study looked at the effect of aluminum nanoparticle additives to toluene. An aerosol shock tube approach was used to evaluate the combustion of a baseline toluene mixture and a toluene/Al nanoparticle mixture with 95% dilution in argon. An aerosol fuel injection method was used to ensure injection of the nanoparticle-doped fuel into the shock tube. The combustion of both the toluene and toluene/Al nanoparticles was determined over the temperature range of 1404 - 1790 K and a pressure around 1.9 atm behind the reflected shock wave. To determine the influence of aluminum nanoparticle additives, the ignition delay times were studied and the results from these experiments are reported. In general, the ignition delay times for temperatures below about 1700 K were not affected by the Al additives, but at higher temperatures the ignition delay times appears to be shorter than for neat toluene. © 2013 by the authors

Self-Poisoning during BH₄^– Oxidation at Pt and Au, and in Situ Poison Removal Procedures for BH₄^– Fuel Cells

Borohydride (BH₄^–) is rapidly attracting attention as an alternative fuel molecule for fuel cells due to its high gravimetric and volumetric energy densities, high power density, and avoidance of the carbon monoxide (CO) poisoning seen in hydrogen (H₂) and methanol (MeOH) fuel cells. Here we describe an as-yet unreported poisoning process that occurs during BH₄^– oxidation at Pt and Au anodes at low potentials (high fuel cell voltages). Though such poisoning can compromise the long-term performance of BH₄^– fuel cells, we also demonstrate an in situ cleaning procedure for a BH₄^– fuel cell using a Pt anode